addCovFromFormla
adds some covariates to a formula;
covs should be a list of variable names
Examples
formla <- y ~ x
addCovToFormla(list("w", "z"), formla)
#> y ~ x + w + z
#> <environment: 0x55c773f75e38>
formla <- ~x
addCovToFormla("z", formla)
#> ~x + z
#> <environment: 0x55c7740452f8>